Solvent can be removed from a polymer solution (or, in other words, polymer can be recovered from a solution) by employing a particle form evaporation process. Particle form evaporation is described, for example, in U.S. Pat. Nos. 4,310,973 and 4,263,091 and in two copending patent applications filed on the same day as this application. W. R. King is the sole inventor named in one of the applications. W. R. King and R. E. Elliott are the co-inventors named in the other application. The above-identified patents are incorporated by reference herein. Broadly, particle form evaporation involves introducing a polymer solution into an agitated bed of polymer particles. Typically, the introduction of polymer solution is accomplished by spraying the polymer solution into the bed. There are many variations of the process compatible with my invention. Accordingly, the following more detailed description of particle bed evaporation is intended to be an illustration and should not be interpreted to unduly limit the scope of my invention.
Generally, the polymer solution is comprised of the polymer and, typically, a hydrocarbon solvent such as, for example, cyclohexane or n-hexane. The polymer solution is usually preheated to a temperature sufficiently high to vaporize a substantial portion of the solvent when it is sprayed into the polymer bed but not so high as to cause thermal degradation of the polymer. In the case of a rubbery polymer solution such as butadiene-styrene copolymer in cyclohexane the solution can be preheated up to about 400.degree. F. (204.degree. C.) without encountering the adverse effects of thermal degradation.
Once the polymer solution has been sufficiently heated, the solution is flashed by spraying it into an agitated bed of dry polymer particles. The flashing of the solvent yields porous droplets having a higher concentration of solids. Generally, 25-70 percent of the solvent is removed in the spray flash depending upon the temperature of the solution, the characteristics of the polymer, the viscosity of the solution, and its concentration. The polymer solution can be concentrated by flashing in one or more stages before it is heated and sprayed into the agitated bed of polymer particles. The droplets, which would otherwise coalesce into a viscous mass, are enveloped by hot, relatively dry polymer particles. Agglomerates having a sticky droplet as a core and a coating of dry polymer on the surface are formed.
The bed of polymer particles provides resistance to the mechanical agitation (e.g. rotating mixer blades). As a result of this resistance there is an input of heat into the bed. The mechanical agitation supplies all or a significant portion of the total heat required for the drying process. Rotating blades can be used to provide the agitation of the polymer bed. The term blade is intended to be broadly construed and includes, for example, paddles, rods, pins and the like.
When more energy is required for the drying process it can be supplied by passing a hot inert gas into and through the polymer bed. The energy imparted to the system from the mechanical agitation and the inert gas causes evaporation of substantially all of the remaining solvent from the polymer.
Although other gases could be used, inert gas is used in order to reduce the safety and health hazards associated with other gases such as, for example, hydrocarbon gases, and to minimize oxidation of the hot polymer. Examples of inert gases which can be used include N.sub.2, CO.sub.2 and fuel gas. The temperature of the inert gas is usually fixed at some temperature above the nomal boiling point of the solvent, but below the point at which significant thermal degradation of the polymer will occur. In the case of drying a butadiene-styrene rubbery copolymer in solution in cyclohexane, an operating temperature of 190.degree.-275.degree. F. is preferred. Since this temperature is above the boiling point of the solvent cyclohexane, the solvent will be vaporized and carried off, yet the polymer will not stick to the equipment since the temperature is either below the softening point of the polymer or the temperature exceeds the softening point only to the extent that the shear forces generated by the agitator are greater than the forces of the polymer causing it to "stick". When at rest the polymer becomes "sticky" and forms a lump. However, when shear forces are applied thereto the polymer tends to form particles. Forces rendering the polymer "sticky" are small compared to the internal shear forces generated by the agitator. Normally "sticky" material remains free flowing in a highly agitated bed, but will block into a lump after it is removed and kept in a static state. At extreme conditions, e.g., operating temperatures that greatly exceed 275.degree. F. in the case of drying a butadiene-styrene copolymer, "stickiness" can become a problem that requires more mechanical shear to overcome.
The temperature of the bed can be controlled by controlling (1) the input of inert gas through the bed of polymer particles, (2) the power input to the agitator, or more preferably (3) the rate at which the polymer solution is added to the bed. Note that the greater the amount of polymer solution added to the bed the lower the temperature of the bed. Conversely, the temperature of the bed will tend to increase as less polymer solution is added. As discussed above, the bed should be maintained at a temperature sufficient to avoid an unacceptable degree of "stickiness." The appropriate temperature of the bed would depend upon such factors as, for example, the polymer and the solvent. If an inert gas is employed, the temperature at which the bed is kept can be substantially the same as the temperature of the inert gas circulating therethrough. For a butadiene-styrene copolymer bed with N.sub.2 as the inert gas, the bed is typically maintained at about 190.degree.-275.degree. F.
Substantially dry polymer particles are removed from the bed and optionally passed to a blower-grinder. The particles can be ground and a portion thereof recycled to the bed to facilitate drying. In this manner a fresh supply of fine, dry polymer particles is always available for the bed. The rate of discharge of polymer from the bed may be controlled by a discharge means such as, for example, a slide valve, which is automatically controlled by the motor load of the mechanical agitator. The greater the load on the motor, the more polymer discharged. The smaller the load on the motor, the less polymer discharged.
Particle form evaporation is most readily applicable to any polymer solution which can yield a flowable crumb in the particle form evaporator at a temperature of about 20.degree.-50.degree. F. (11.degree.-28.degree. C.) above the normal boiling point of the solvent.
Some polymers such as, for example, the polybutadienes, are so fluid that even with agitation the tendency towards formation of particles is so weak that the particle form evaporation process can be impracticable as a method for drying the polymer (i.e. removing solvent from the polymer). Successful removal of substantially all of the solvent depends upon creation and maintenance of small polymer particles. My invention makes practicable application of the particle form evaporation process to such polymers and facilitates the processing of polymers already suitable for the particle form evaporation process.